The present invention relates to heating a geological formation for the extraction of hydrocarbons. In particular, the present invention relates to an advantageous applicator, system, and method that can be used to heat a geological formation to extract heavy hydrocarbons.
As the world's standard crude oil reserves are depleted and the continued demand for oil causes oil prices to rise, oil producers are attempting to process hydrocarbons from bituminous ore, oil sands, tar sands, and heavy oil deposits. These materials are often found in naturally occurring mixtures of sand or clay. Because of the extremely high viscosity of bituminous ore, oil sands, oil shale, tar sands, and heavy oil, the drilling and refinement methods used in extracting standard crude oil are typically not available. Therefore, recovery of oil from these deposits requires heating to separate hydrocarbons from other geologic materials and maintaining hydrocarbons at temperatures at which they will flow.
Current technology heats the hydrocarbon formations through the use of steam and sometimes through the use of electric or radio frequency heating. Steam has been used to provide heat in-situ, such as through a steam assisted gravity drainage (SAGD) system. Steam enhanced oil recovery (EOR) may require caprock over the hydrocarbon formations to contain the steam. The use of steam in permafrost regions may be problematic because it can melt the permafrost along the well near the surface.
RF heating is heating using one or more of three energy forms: electric currents, electric fields, and magnetic fields at radio frequencies. Depending on operating parameters, the heating mechanism may be resistive by joule effect or dielectric by molecular moment. Resistive heating by joule effect is often described as electric heating, where electric current flows through a resistive material. Dielectric heating occurs where polar molecules, such as water, change orientation when immersed in an electric field. Magnetic fields also heat electrically conductive materials through eddy currents, which heat resistively.
RF heating can use electrically conductive antennas to function as heating applicators. The antenna is a passive device that converts applied electrical current into electric fields, magnetic fields, and electrical current fields in the target material without having to heat the antenna structure to a specific threshold level. Preferred antenna shapes can be Euclidian geometries, such as lines and circles. Additional background information on dipole antennas can be found at Antennas: Theory and Practice by S. K. Schelkunoff and H. T. Friis, Wiley New York, 1952, pp 229-244, 351-353. The radiation patterns of antennas can be calculated by taking the Fourier transform of the antenna's electric current flow. Modern techniques for antenna field characterization may employ digital computers and provide for precise RF heat mapping.
Antennas can be made from many things including Litz conductors. Litz conductors are often composed of wire rope which can reduce resistive losses in electrical wiring. Each of the conductive strands used to form the Litz conductor has a nonconductive insulation film over it. The individual stands may be about 1 RF skin depth in diameter at the frequency of usage. The strands are variously bundled, twisted, braided or plaited to force the individual strands to occupy all positions in the cable. In this way the current must be shared equally between strands. Thus, Litz conductors reduce the ohmic losses by reducing the RF skin effect in electrical wiring. Litz conductors are sometimes known as Litzendraught conductors and the term may relate to “lace telegraph wire” in German.
U.S. Pat. No. 7,205,947 entitled “Litzendraught Loop Antenna and Associated Methods” to Parsche describes a wire loop antenna of Litz conductor construction. The strands are severed at intervals to introduce distributed capacitance for tuning purposes and the Litz conductor loop is fed inductively from a second nonresonant loop.